The mammalian heart is understood to maintain an intrinsic rhythm by creating electric stimuli. Generally, the stimuli form a depolarization wave that propagates within specialized cardiac conducting tissue and the myocardium. The usually well-ordered wave movement facilitates coordinated contractions of the myocardium. These contractions are the engine that moves blood throughout the body. See generally The Heart and Cardiovascular System. Scientific Foundations. (1986) (Fozzard, H. A. et al. eds) Raven Press, NY.
Under most circumstances, cardiac stimuli are controlled by recognized physiological mechanisms. However there has been long-standing recognition that abnormalities of excitable cardiac tissue can lead to abnormalities of the heart rhythm. These abnormalities are generally referred to as arrhythmias. Most arrhythmias are believed to stem from defects in cardiac impulse generation or propagation that can substantially compromise homeostasis, leading to substantial patient discomfort or even death. For example, cardiac arrhythmias that cause the heart to beat too slowly are known as bradycardia, or bradyarrhythmia. In contrast, arrhythmias that cause the heart to beat too fast are referred to as tachycardia, or tachyarrhythmia. See generally Cardiovascular Arrhythmias (1973) (Dreifus, L. S. and Likoff, W. eds) Grune & Stratton, NY.
The significance of these and related heart disorders to public health cannot be exaggerated. Symptoms related to arrhythmias range from nuisance, extra heart beats, to life-threatening loss of consciousness. Complete circulatory collapse has also been reported. Morbidity and mortality from such problems continues to be substantial. In the United States alone for example, cardiac arrest accounts for 220,000 deaths per year. There is thought to be more than 10% of total American deaths. Atrial fibrillation, a specific form of cardiac arrhythmia, impacts more than 2 million people in the United States. Other arrhythmias account for thousands of emergency room visits and hospital admissions each year. See eg., Bosch, R. et al. (1999) in Cardiovas Res. 44: 121 and references cited therein.
Cardiac electrophysiology has been the subject of intense interest. Generally, the cellular basis for all cardiac electrical activity is the action potential (AP). The AP is conventionally divided into five phases in which each phase is defined by the cellular membrane potential and the activity of potassium, chloride, and calcium ion channel proteins that affect that potential. Propagation of the AP throughout the heart is thought to involve gap junctions. See Tomaselli, G. and Marban, E. (1999) in Cardiovasc. Res. 42: 270 and references cited therein.
There have been limited attempts to treat cardiac arrhythmias and related heart disorders.
Specifically, many of the past attempts have been confined to pharmacotherapy, radiofrequency ablation, use of implantable devices, and related approaches. Unfortunately, this has limited options for successful patient management and rehabilitation.
In particular, radiofrequency ablation has been reported to address a limited number of arrhythmias eg., atrioventricular (AV) node reentry tachycardia, accessory pathway-mediated tachycardia, and atrial flutter. However, more problematic arrhythmias such as atrial fibrillation and infarct-related ventricular tachycardia, are less amenable to this and related therapies. Device-based therapies (pacemakers and defibrillators, for instance) have been reported to be helpful for some patients with bradyarrhythmias and lifesaving for patients with tachyarrhythmias. However, such therapies does not always prevent tachyarrhythmias. Moreover, use of such implementations is most often associated with a prolonged commitment to repeated procedures, significant expense, and potentially catastrophic complications including infection, cardiac perforation, and lead failure.
Drug therapy remains a popular route for reducing some arrhythmic events. However, there has been recognition that systemic effects are often poorly tolerated. Moreover, there is belief that proarrhythmic tendencies exhibited by many drugs may increase mortality in many situations. See generally Bigger, J. T and Hoffman, B. F. (1993) in The Pharmacological Basis of Therapuetics 8th Ed. (Gilman, A. G et al. eds) McGraw-Hill, NY and references cited therein.
It would be desirable to have more effective methods for treating or preventing cardiac arrhythmias. It would be especially desirable to have gene therapy methods for treating or preventing such arrhythmias.